This invention relates to an air-to-fuel feedback control system wherein the air-to-fuel ratio of the mixture represented by the concentration of oxygen in the exhaust gases from an internal combustion engine, is fed back to correct the air-to-fuel ratio of the mixture.
Heretofore, it has been proposed to detect and feed back the oxygen content of the exhaust gases from an internal combustion engine to correct the air-to-fuel ratio of the mixture and thereby reduce the emissions of harmful constituents in the exhaust gases, and this control through the feedback system is particularly advantageous in that the air-to-fuel ratio can be controlled at the desired value against considerably large variations in the external conditions, e.g., variations in the atmospheric pressure, variations in the intake air temperature, etc. However, the air-to-fuel ratio detectors used for this control are of the type whose operating conditions change with temperature, and particularly when the temperature of exhaust gases is low (lower than about 350.degree. to 400.degree. C), such detector continuously produces a constant output voltage irrespective of the oxygen content changes in the exhaust gases. Consequently, it is undesirable to effect feedback correction of the air-to-fuel ratio of the mixture in accordance with the output voltage of the detector which is produced during such inoperable period.